Personally I'd
1. Get the cheapest old-stock copy of Photoshop 7 you can find. It's the oldest version that qualifies for CS3 upgrade status and you can probably pick it up for peanuts on ebay.
2. Get a legit boxed Photoshop CS3 upgrade, which if you buy from a US store and get it shipped to the UK will cost you about 100 quid ($200) given the current exchange rate.
Don't bother with Photoshop Extended. It's unlikely to have anything you'll need and will just cost more. You could do something similar with Flash.